As a membrane separation device of this type, an immersion type membrane separation device having plural membrane elements disposed parallel to each other with a given interval (5-10 mm) is known. These membrane elements each are made up of, for example, a plate for filtration that is made of a resin and has a rectangular flat plate shape, defining a filtered water flow passage that has a first end opening to the surface of the plate and a second end communicating to a filtered water suction conduit, an organic filtration membrane covering the surface of this resin plate, and a spacer disposed between the plate and the organic filtration membrane to have a given clearance.
The membrane separation device has membrane units each made up of a plurality of the aforesaid membrane elements that have the filtered water suction conduits connected together to have a common conduit, and these membrane units are immersed in water to be treated within a treated water tank, in which a negative pressure is applied to the filtered water flow passage to filter the water to be treated by an organic filtration membrane, thus obtaining filtered water. In the aforesaid membrane separation device, an air diffuser for generating air bubbles is disposed in a lower portion of each membrane unit, so that air bubbles generated move upward between the membrane elements to generate a cross-flow. This cross-flow removes cake layer formed on the membrane surface as the filtration progresses. This membrane separation device can deal with various filtration volumes by increasing or decreasing the number of membrane elements or increasing or decreasing the effective membrane area, and thus can be used for various purposes from a small scale filtration to a large scale wastewater treatment plant.
These membrane elements are subjected to pressure by the aforesaid cross-flow and therefore a measure to prevent the organic filtration membrane, which is disposed over the surface of a resin plate for filtration, from being peeled off by fixing the organic filtration membrane to a peripheral portion of the resin plate by adhesive has been employed. However, according to this fixing manner, the organic filtration membrane is fixed to the resin plate by having adhesive impregnated in nonwoven fabric that acts as a substrate of the organic filtration membrane. Therefore, the fixing strength is varied depending on the strength, durability or chemical resistance of the cured adhesive and there may be a problem in that the fixing strength is relatively low, the working environment is deteriorated by solvent, or the drying and curing takes time. Also, there has been used a method in which the fixing is made by a tape, but this physical fixing poses a problem in that the filter membrane is easy to be peeled off. Therefore, there have been proposed manufacturing methods disclosed in such as in Japanese Patent No. 3028900 and Japanese Patent Application Laid-open No. 2001-120958.    Patent Document 1: Japanese Patent No. 3028900    Patent Document 2: Japanese Patent Application Laid-open No. 2001-120958    Patent Document 3: Japanese Patent Application Laid-open No. Hei-5-68943 (page 3 paragraph [0002])    Patent Document 4: Japanese Patent Application Laid-open No. Sho-58-30378 (page 2 right upper column line 15 to left lower column line 7)
The aforesaid Japanese Patent No. 3028900 discloses a method in which a thermoplastic resin plate and an organic filtration membrane are fusion bonded together by ultrasonic wave. The aforesaid Japanese Patent Application Laid-open No. 2001-120958 discloses a method in which plural protrusions are formed on a thermoplastic resin plate of a fusion bonded portion to have a difference in fusing strength by the protrusions, thereby suppressing the occurrence of fatigue cracking due to heat applied to a microporous organic filter membrane during fusion bonding.
Japanese Patent No. 3028900 discloses in paragraph [0010] that nonwoven fabric made of such as saturated polyester is used as a substrate of a filtration membrane 2; and when the temperature in fusion bonding this substrate to a plate for filtration 1 made of such as an ABS resin by ultrasonic wave is lower than 140° C., the plate 1 is fused and the fused resin is impregnated into the nonwoven fabric so that the filtration membrane 2 can be fixed to the plate 1; and when the temperature is higher than 140° C., both the plate 1 and the nonwoven fabric are fused so that the filtration membrane 2 can be fixed to the plate 2. However, in the fusion bonding by ultrasonic wave, which tends to cause uneven heating, controlling the temperature to below 140° C. is not preferable from the view point of securing a stable performance of a membrane element, and therefore it is assumed that the temperature was controlled to above 140° C. This is also apparent from Japanese Patent Application Laid-open No. 2001-120958 that discloses that nonwoven fabric made of synthetic resin fibers, which acts as a substrate, is partially fused and hence fatigue cracking is caused during the ultrasonic fusion bonding. Japanese Patent Application Laid-open No. 2001-120958 discloses a prior art in paragraphs [0007] to [0008] and in FIGS. 10 and 11, in which a filtration membrane 22B is pressed to a plate for filtration 22A by a rotating rotary horn 31 and fusion bonding is made by using ultrasonic wave, and the filtration membrane 22B is fusion bonded to the plate 22A by fusing the plate 22A thereby deforming the same into groove-like recesses in a water shutoff portion S. Since it discloses that fatigue cracking is easy to occur, it is assumable that the nonwoven fabric of synthetic resin fibers, which acts as the substrate, is partially fused.